The Zintl phase Eu2Si

Article abstract of DOI:10.1002/1521-3749(200205)628:4<741::AID-ZAAC741>3.0.CO;2-1

The Zintl phase Eu₂Si was synthesized from elemental europium and silicon in a sealed tantalum tube in a high-frequency furnace at 1270 K and subsequent annealing at 970 K. Investigation of the sample by X-ray powder and single crystal techniques revealed: Co₂Si (anti-PbCl₂) type, space group Pnma, a = 783.0(1), b = 504.71(9), c = 937.8(1) pm, wR2 = 0.1193, 459 F² values and 20 variables. The structure contains two europium and one silicon site. ¹⁵¹Eu Moessbauer spectroscopic data show a single signal at an isomer shift of -9.63(3) mm/s, compatible with divalent europium. Within the Zintl concept electron counting can be written as (2Eu²⁺)⁴⁺Si^4-, in agreement with the absence of Si-Si bonding. Each silicon atom has nine europium neighbors in the form of a tri-capped trigonal prism. The silicon coordinations of the Zintl phases Eu₂Si, Eu₅Si₃, EuSi, and EuSi₂ are compared.

Full text of DOI:10.1002/1521-3749(200205)628:4<741::AID-ZAAC741>3.0.CO;2-1

The Zintl Phase Eu Si
2
a
b
b,*
c
c
Ratikanta Mishra , Rolf-Dieter Hoffmann , Rainer Pöttgen , Henning Trill , and Bernd D. Mosel
a
Mumbai/India, Applied Chemistry Division, B.A.R.C. Münster, b Institut für Anorganische und Analytische Chemie and c Institut für
Physikalische Chemie der Westfälischen Wilhelms-Universität
Received November 22nd, 2001.
2
Abstract. The Zintl phase Eu Si was synthesized from elemental
isomer shift of Ϫ9.63(3) mm/s, compatible with divalent europium.
europium and silicon in a sealed tantalum tube in a high-frequency
furnace at 1270 K and subsequent annealing at 970 K. Investiga-
tion of the sample by X-ray powder and single crystal techniques
Within the Zintl concept electron counting can be written as
2
ϩ 4ϩ 4Ϫ
(2Eu
) Si , in agreement with the absence of Si-Si bonding.
Each silicon atom has nine europium neighbors in the form of a
tri-capped trigonal prism. The silicon coordinations of the Zintl
phases Eu Si, Eu Si , EuSi, and EuSi are compared.
2 5 3 2
revealed: Co
b ϭ 504.71(9), c ϭ 937.8(1) pm, wR2 ϭ 0.1193, 459 F values and
0 variables. The structure contains two europium and one silicon
2 2
Si (anti-PbCl ) type, space group Pnma, a ϭ 783.0(1),
2
2
151
site. Eu Mössbauer spectroscopic data show a single signal at an
Keywords: Zintl Phases; Crystal structure; Europium
Die Zintl-Phase Eu Si
2
Inhaltsübersicht. Die Zintl-Phase Eu
ropium und Silicium in einer verschweißten Tantalampulle im
Hochfrequenzofen bei 1270 K synthetisiert und anschließend bei
2
Si wurde aus elementarem Eu-
151Eu Mössbauer-spektroskopische Daten zeigen nur ein Signal bei
einer Isomerieverschiebung von Ϫ9.63(3) mm/s, was mit zweiwerti-
gem Europium kompatibel ist. Im Rahmen des Zintl-Konzepts
2
ϩ 4ϩ 4Ϫ
9
70 K getempert. Untersuchungen der Probe mit Röntgen-Pulver-
und Einkristalldaten ergab: Co Si (anti-PbCl )-Typ, Raumgruppe
Pnma, a ϭ 783,0(1); b ϭ 504,71(9); c ϭ 937,8(1) pm; wR2 ϭ
kann die Formel als (2Eu ) Si geschrieben werden, in Über-
2
2
einstimmung mit der Abwesenheit von SiϪSi-Bindungen. Jedes Si-
liciumatom hat neun Europiumnachbarn in Form eines dreifach
überkappten trigonalen Prismas. Die Siliciumkoordination in den
2
0
,1193; 459 F -Werte und 20 variable Parameter. Die Struktur ent-
hält zwei kristallographisch unterschiedliche Europiumplätze.
2 5 3 2
Zintl-Phasen Eu Si, Eu Si , EuSi, und EuSi wird verglichen.
We have recently started a more systematic study on ter-
nary rare earth metal iridium silicides [15, 16]. Studies in
the europium based system revealed a presumably new
compound. Investigation by X-ray powder and single crys-
tal data indeed revealed that it is a binary silicide. Herein
we report on the synthesis and structure refinement of the
Introduction
The binary system europiumϪsilicon is so far characterized
by the silicides Eu Si (Cr B type, space group I4/mcm) [1,
5
3
5 3
2
], EuSi (CrB type, space group Cmcm) [3Ϫ8], and EuSi2
(
α-ThSi type, space group I4 /amd) [5Ϫ14]. These silicides
2
1
contain purely divalent europium and chemical bonding
Zintl phase Eu Si, the so far most metal-rich phase in the
2
can be rationalized by the ZintlϪKlemm concept:
binary system europium-silicon.
5Eu2
ϩ 10ϩ
)
[Si2 ][Si ], Eu Si and Eu [2Si ]. Accor-
6Ϫ
4Ϫ
2ϩ 2Ϫ
2ϩ
Ϫ
(
4
Ϫ
2Ϫ
ding to the electron count the Si anions are isolated, Si
forms zig-zag chains, Si a three-dimensional network with
three-connected silicon and the Si2 dumb-bells are iso-
electronic with chlorine. During a systematic study of the
EuX (X ϭ Si, Ge, Sn) tetrelides Loewenhaupt [5] reported
on a new binary silicide with the approximate composition
Ϫ
Experimental
6
Ϫ
2
Starting materials for the preparation of Eu Si were sublimed pieces
of europium (Johnson-Matthey) and silicon lumps (Wacker)
both with stated purities better than 99.9 %. The moisture sensitive
europium pieces were kept in Schlenk tubes under argon prior to
the reactions. The argon (Messer-Griesheim 4.8) was purified by
titanium sponge (870 K), molecular sieves, and silica gel. Small eu-
ropium pieces were mixed with silicon powder in the ideal 2:1
atomic ratio (total weight ca. 800 mg) and sealed in a small tanta-
ϳEu Si with a yet unknown structure. This phase, how-
4
5
ever, has not been reported in other studies.
3
lum tube (volume ca. 1 cm ) under an argon pressure of about
*
Prof. Dr. R. Pöttgen
8
00 mbar. Details concerning the arc-welding equipment are given
Institut für Anorganische und Analytische Chemie
Westfälische Wilhelms-Universität Münster
Wilhelm-Klemm-Straße 8
D-48149 Münster
Fax: (ϩ49)-(0)251-83-36002
elsewhere [17].
The tube was subsequently placed in a water-cooled sample cham-
ber [18] of a high-frequency furnace (Hüttinger TIG 1.5/300) and
first heated under flowing argon at about 1270 K for one minute.
The reaction between europium and silicon was indirectly visible
E-mail: pottgen@uni-muenster.de
Z. Anorg. Allg. Chem. 2002, 628, 741Ϫ744
 WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 0044Ϫ2313/02/628/741Ϫ744 $ 20.00ϩ.50/0 741

R. Mishra, R.-D. Hoffmann, R. Pöttgen, H. Trill, B. D. Mosel
2
Table 1 Crystal data and structure refinement for Eu Si.
Empirical formula
Formula weight
Unit cell dimensions
2
Eu Si
332.01 g/mol
a ϭ 783.0(1) pm
b ϭ 504.71(9) pm
c ϭ 937.8(1) pm
(powder data)
3
V ϭ 0.3706 nm
Space group
Pnma (No. 62)
Z ϭ 4
5.95 g/cm
Formula units per cell
Calculated density
Crystal size
3
3
10 ϫ 10 ϫ 40 µm
Transmission ratio (max/min)
Absorption coefficient
F(000)
Detector distance
Exposure time
1.49
33.6 mm
560
50 mm
Ϫ1
20 min
φ range, increment
Profile / pixel
θ range for data collection
Range in hkl
Total no. reflections
Independent reflections
Reflections with IϾ2σ(I)
Data/parameters
0Ϫ160.5° / 1.5°
11Ϫ13
3° to 28°
±10, ±5, ±12
2623
459 (Rint ϭ 0.1383)
273 (Rsigma ϭ 0.1288)
459 / 20
Fig. 1 Scanning electron micrograph of a freshly crushed surface
of a Eu Si fragment.
2
2
Goodness-of-fit on F
0.872
Final R indices [IϾ2σ(I)]
R indices (all data)
Extinction coefficient
Largest diff. peak and hole
R1 ϭ 0.0487, wR2 ϭ 0.1099
R1 ϭ 0.0822, wR2 ϭ 0.1193
0.009(1)
Table 2 Atomic coordinates and anisotropic displacement para-
meters (pm ) for Eu Si. All atoms are situated on the Wyckoff posi-
2
tion 4c (x 1/4 z) of space group Pnma. Ueq is defined as one third
of the trace of the orthogonalized Uij tensor. U23 ϭ U12 ϭ 0.
2
˚
3
2.27 and Ϫ2.24 e/A
Atom
x
z
U
11
U
22
U
33
U
13
U
eq
Eu1
Eu2
Si
0.9810(2)
0.3439(2)
0.247(1)
0.6806(2)
0.4216(2)
0.1000(9)
225(10)
284(10)
253(44)
258(12)
176(11)
147(53)
247(8)
205(8)
222(42) Ϫ14(34)
Ϫ4(7)
3(7)
243(6)
222(6)
207(18)
through a heat flash within the sample chamber. The tube was
subsequently annealed at 970 K for another hour. After the an-
nealing procedure the silvery sample could easily be separated from
the tube. Eu
Schlenk tubes under dry argon.
Eu Si was characterized through its powder diffractogram (Stoe
Stadi P) using CuKα radiation and silicon (a ϭ 543.07 pm) as an
external standard. The lattice parameters (Table 1) were obtained
from a least-squares fit of the powder data. The correct indexing
was ensured by an intensity calculation [19] taking the atomic posi-
tions from the structure refinement. The lattice parameters deter-
mined from the powder and the single crystal agreed well. In all
2
Si is moisture sensitive. The sample was kept in
2
Table 3 Interatomic distances (pm), calculated with the lattice pa-
rameters taken from X-ray powder data of Eu Si. All distances
shorter than 504 pm (Eu-Eu), 445 pm (Eu-Si), and 482 pm (SiϪSi)
1
2
are listed. Standard deviations are given in parentheses.
Eu1: 1 Si
320.5(9) Eu2: 1 Si
310.9(9) Si: 1 Eu2 310.9(9)
2
2
2
2
1
1
2
2
Si
338.6(6)
2
1
2
2
1
2
1
Si
Si
311.1(5)
316.5(9)
2
1
1
2
2
Eu2 311.1(5)
Eu2 316.5(9)
Eu1 320.5(9)
Eu1 338.6(6)
Eu1 371.5(7)
5 3
powder patterns a small amount of Eu Si was observed. The ratio
Eu2 365.4(2)
Eu2 370.9(2)
Si
Eu2 373.8(2)
Eu2 388.3(2)
Eu1 412.6(1)
Eu1 423.4(3)
Eu Si/Eu Si was estimated to about 90/10. Most likely, a small
2
5
3
Eu1 365.4(2)
Eu1 370.9(2)
Eu1 373.8(2)
Eu2 380.9(3)
Eu1 388.3(2)
amount of europium distilled to the colder part of the tantalum
tube, leading to the europium poorer phase. This behavior was also
observed for samples with the starting composition 2.5Eu/1Si.
A freshly crushed surface of the sample and the composition were
studied in a scanning electron microscope (LEICA 420 I) using
371.5(7)
2 3
SiO and EuF as standards. A typical cleaved surface (Figure 1)
shows lamellar crystal growth. Energy dispersive analyses of the
larger platelets revealed compositions very close to the ideal one
(
Eu
2
Si).
graphic data and experimental details for the data collection are
listed in Table 1.
An irregularly shaped single crystal of Eu
2
Si was isolated from the
annealed sample by mechanical fragmentation and examined by
Laue photographs on a Buerger camera in order to establish suita-
bility for intensity data collection. Single crystal intensity data were
collected at room temperature by use of a Stoe image plate system
The starting atomic parameters were deduced from an automatic
interpretation of direct methods with SHELXS-97 [21] and the
structure was successfully refined using SHELXL-97 (full-matrix
2
least-squares on F ) [22] with anisotropic atomic displacement pa-
(
(
IPDS) with graphite monochromatized MoKα radiation
71.073 pm) in oscillation mode. A numerical absorption correction
rameters for all atoms. As a check for the correct composition, the
occupancy parameters were refined in a separate series of least-
squares cycles. All sites were fully occupied within two standard
deviations and in the final cycles the ideal occupancies were as-
sumed again. A final difference Fourier syntheses revealed no signi-
ficant residual peaks (see Table 1). The positional parameters and
was applied to the data.
2
Isotypism of Eu Si with the orthorhombic PbCl type [20] was al-
ready evident from the powder data. Space group Pnma was found
2
to be correct during the structure refinement. All relevant crystallo-
742
Z. Anorg. Allg. Chem. 2002, 628, 741Ϫ744

2
The Zintl Phase Eu Si
2
Fig. 2 Projection of the Eu Si structure onto the xz plane. All
atoms lie on mirror planes at y ϭ 1/4 (thin lines) and y ϭ 3/4 (thick
lines), respectively. The europium and silicon atoms are drawn as
open and filled circles. The trigonal prisms around the silicon
atoms are emphasized.
interatomic distances are listed in Tables 2 and 3. Listings of the
1)
observed and calculated structure factors are available .
151
The 21.53 keV transition of Eu with an activity of 130 MBq (2 %
of the total activity of a 151Sm:EuF
source) was used for the Möss-
3
bauer spectroscopic experiments. The measurements were perform-
ed with a commercial helium bath cryostat. The temperature of
the absorber was kept at 78 K. The source was kept at room temper-
ature. The sample was placed within a thin-walled PVC container
Fig. 3 Coordination of the silicon atoms in the Zintl phases
Eu Si, Eu Si , EuSi, and EuSi . The europium and silicon atoms
2
5
3
2
are drawn as open and filled circles, respectively. Atom designations
and relevant distances (in units of pm) are indicated.
2
at a thickness corresponding to about 10 mg Eu/cm .
belong to the coordination sphere of Eu1. Both europium
sites have three and four silicon neighbors, respectively, at
Eu-Si distances ranging from 311 to 339 pm, comparable to
the sum of the atomic radii of 321 pm [37]. Two further
silicon neighbors occur at the much longer Eu1ϪSi distance
of 372 pm for the Eu1 atoms. Also in this case these atoms
belong to the Eu1 coordination sphere.
Results and Discussion
The Zintl phase Eu Si is the so far most metal-rich com-
2
pound in the binary system Eu/Si. Eu Si adopts the ortho-
rhombic anti-PbCl2 [20] (Co Si [23]) type structure. This
structure also occurs for the isotypic europium and ytter-
bium based Zintl phases Eu Sn [24] and Yb Sn [25], and
2
2
2
2
the alkaline earth (AE) metal based compounds AE X
2
Although the sample consisted of a mixture of approxi-
(AE ϭ Ca, Sr, Ba; X ϭ Si, Ge, Sn, Pb) [26Ϫ35].
151
mately 90 % Eu Si and 10 % Eu Si we recorded a
Eu
2
5
3
A projection of the Eu Si structure onto the xz plane
is presented in Figure 2. The silicon atoms have a slightly
distorted trigonal prismatic europium coordination (Figure
2
Mössbauer spectrum at 78 K (Figure 4) in order to get in-
formation on the valence state of the europium atoms. We
observed
a single signal at an isomer shift δ ϭ
3
). The coordination sphere is augmented by three further
Ϫ9.63(3) mm/s with an experimental line width of Γ ϭ
europium atoms which are situated on the rectangular faces
of these prisms (tri-capped trigonal prism). Within the x
direction the prisms are condensed via common edges while
they share common triangular faces along the y direction.
The various EuϪEu interactions range from 365 to
2
.7(1) mm/s and an electric field gradient ∆EQ
ϭ
9
.0(6) mm/s. Also an asymmetry parameter η ϭ 0.7(1) was
refined. Our recent susceptibility data of Eu Si [1] revealed
divalent europium. In view of the absence of a signal
around 0 mm/s we can assume purely divalent europium in
5
3
3
88 pm. In view of the EuϪEu distances of 397 pm in ele-
Eu Si. The experimentally observed line width of
2
mental bcc europium [36] and the electron transfer from
europium to silicon, these contacts may only be considered
as weakly bonding. The longer Eu1ϪEu1 contacts (413 and
2
2
.7(1) mm/s is slightly larger than the usual line width of
.3 mm/s for europium. This slight enlargement, and most
likely also the asymmetry parameter, certainly account for
the mixture Eu Si / Eu Si . Based on the Eu Mössbauer
spectroscopic data we can assume that the europium atoms
4
23 pm) may not be considered as bonding, however, from
151
2
5
3
a geometrical point of view, these additional Eu1 atoms still
have transferred two valence electrons to the silicon atoms.
Electron counting can then be written as (2Eu2
) Si
ϩ 4ϩ 4Ϫ
1
)
classifying Eu Si as a Zintl phase.
In Figure 3 we compare the silicon coordinations of the
various binary Zintl phases in the europiumϪsilicon sys-
2
Details may be obtained from: Fachinformationszentrum Karls-
ruhe, D-76344 Eggenstein-Leopoldshafen (Germany), by quoting
the Registry No. CSD-412248.
Z. Anorg. Allg. Chem. 2002, 628, 741Ϫ744
743

R. Mishra, R.-D. Hoffmann, R. Pöttgen, H. Trill, B. D. Mosel
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[
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5
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1
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3
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2
2
type, i.e. tri-capped trigonal prims. In all cases the trigonal
prisms are formed by the europium atoms. Depending on
the composition of the silicide, the atoms on the rectangular
faces are europium atoms for the most metal-rich silicide
[
(
Eu Si) and silicon atoms for the silicon-rich silicide
2
[
(
EuSi ). In Eu Si [1, 2] and EuSi [3Ϫ8] we observe euro-
2
5
3
1
0, 73.
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5
3
[
[
20] K. Sahl, Beitr. Mineral. Petrogr. 1963, 9, 111.
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[
Summing up, we have prepared the Zintl phase Eu Si
2
1
51
with anti-PbCl type structure.
Eu Mössbauer spectro-
2
scopic data revealed divalent europium leading to an elec-
tron count (2Eu2
ϩ 4ϩ 4Ϫ
)
Si . In the corresponding germa-
[
[
[
[
25] P. Manfrinetti, D. Mazzone, A. Palenzona, J. Alloys Compd.
999, 284, L1.
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2
1
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3
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Göcke for the work at the electron microscope. This work was fi-
nancially supported by the Fonds der Chemischen Industrie and
the Deutsche Forschungsgemeinschaft. R. M. is indebted to the
Alexander von Humboldt Foundation for a research grant.
3
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Z. Anorg. Allg. Chem. 2002, 628, 741Ϫ744